During rolling in a rolling mill, the thickness of the material layer to be rolled significantly influences the efficiency of the grinding. A thin material layer normally leads to a more efficient grinding and consequently an improved grinding economy. However, the thickness of the grinding cushion must not be too small, inasmuch as this would involve the risk of direct contact between the grinding path and the rollers. In turn, this would cause severe impacts with a consequent danger of damaging the mill installation. Thus, it is sought to maintain the thickness of the grinding cushion at a certain constant value during variable working conditions.
The thickness of the grinding cushion is determined by the amount of feed and by the pressure with which the rollers are pressed towards the grinding path as well as by the granulometry, grindability, and humidity of the material fed.
In small mills the working pressure may be obtained by means of springs which provide for pressing the rollers towards the grinding path. In such mills, the thickness of the grinding cushion will adapt itself, depending upon the amount and properties of the material fed, to a thickness alllowing the springs to yield and provide the necessary grinding pressure. When the thickness of the grinding cushion has adjusted itself, any increase in the thickness will bring about an increased working pressure which will tend to reestablish the initial value of the thickness of the grinding cushion. Correspondingly, if the thickness of the grinding cushion is decreased, this will lead to a lowering of the working pressure.
In large mills, however, the grinding pressure cannot be obtained by means of springs of a reasonable size. Consequently, the rollers are pressed towards the grinding path by hydraulic means. This makes it possible to set the working pressure at a desired value, but such a present working pressure still does not avoid variations of the grinding cushion resulting from variations in the feed rate and grindability of the material. German publication DT OS No. 26 35 244 discloses controlling the hydraulics providing the grinding pressure in such a manner that the hydraulic system acts like a spring, i.e., that the working pressure increases or decreases proportionally with the deviation of the grinding cushion thickness from a mean value. In this manner, an adaptation of the grinding pressure is obtainable in regard to the grindability of the material, but at the expense of appreciable variation in the thickness of the grinding cushion.
I have invented an improved method of maintaining the thickness of the grinding cushion of a roller mill at a substantially constant predetermined value as well as an improved roller mill for practicing my improved method.